1. Field of the Invention
The present invention relates to a fault detection apparatus for detecting a closed-circuit fault of a switching element in a bridge circuit by a switching element driving a direct-current motor.
2. Description of the Related Art
When forward-rotating, a direct-current drive bridge circuit of a direct-current motor turns off both of a pair of reverse rotation switching elements, and sets one of a pair of forward rotation switching elements to a full-time on state and executes an on-off duty control of another.
Further, when reverse rotating the direct-current motor, the circuit turns off both of a pair of forward rotation switching elements, and sets one of a pair of reverse rotation switching elements to a full-time on state and executes an on-off duty control of another, in the reverse manner.
In this case, if the other switching elements, other than a pair of switching elements under operation, generate the closed-circuit fault, electric current is increased. As a result an electric current detection resistance is interposed to detect an excess current. However, there are problems. The electric current detection resistance dissipates electric energy. Further, costs are increased because the electric current detection resistance is expensive.
A method of detecting a maximum value and a minimum value of an electric potential has been proposed in two node points to the forward rotation switching element and the reverse rotation switching element, and detecting the closed-circuit fault of the switching elements by comparing with a threshold value (for example, refer to Japanese Patent No. 3020038). However, because the electric potential maximum value detection circuit and an electric potential minimum value detection circuit are provided in each of two node points to the forward rotation switching element and the reverse rotation switching element, and a means for comparing them is also provided. The circuit is complex and has many parts which increases the cost.